I've just had a quite go as them on medium difficulty and hard campaign (my usual settings). What I did was this:
~Turn 1~
Glasobrin > attacks Pessinus
Sennias > attacks Nicomedia
Brennus > attacks Prusa
One spear warband stays behind to garrison Ancrya set on very high tax, I start building the high kings hall and recruit one skirmisher warband.
~Turn 2~
Ancrya riots. Settlement brought under control.
Adopted a general who joined with
Brennus.
Brennus attacks Prusa. Chosen spearmen lead the assault and take on the phalangites head on whilst
Brennus and any other cavalry attack flanks. Combined they will destroy the phalangites. The peltests get attack by everything (again, chosen spearman attacking first).
Occupy settlement.
Adopted general and army move adjacent to Nicomedia.
Sennias attacks Nicomedia. All slingers run into position (even reinforcements) to provide support to battering rams. Chosen spearmen unit goes in through gate alone set on defence mode. When enemy units surround chosen spearmen, galatian swordmen charge into enemy. Rest is simple mop up operation.
Occupy settlement.
Sennias and army move to support
Brennus. Adopted general and army garrison Nicomedia.
Glasobrin attacks Pessinus. Chariots and heavy cavalry are used to early on during fight on plaza. Everything used to take city.
Occupy settlement.
At the end of all that, I disbanded any surviving chosen spearmen and chariots.
My cities had incomes of 1250, 390, 686 and -591. My tresuary JUST stayed in the green at about 28.
If you can replicate what I did though, you are sorted and should be able to fend of Antiochus and his elephants (maybe?

).
The trick is to make effective use of your
chosen spearmen, they are your strongest units and it is a mistake to disband them straight away, because you will then have to recruit galatian swordsmen which in actuality cost you more over those few turns!
An example. Disbanding two units of chosen spearmen = +£1600 per turn. Recruit one galatian swords for ~ £1150 + £450 upkeep. Next turn, £1150 expended. Upkeep increases to £900. So your £3200 saved actually nullified by am expenditure of £1150 + £1150 + £900 + 450 = £3650!!
And you get lesser troops for your troubles!!
Build mines at every opportunity you get. The extra £500 you get from them each turn is a major boost early on.